In the field of large cold storage freezers and similar devices, various systems such as solid doors, strip curtains, and air curtains, may be used to separate the cold storage room from an adjacent relatively warm anteroom. It is desirable to allow traffic from people and equipment through a doorway between the cold storage room and the adjacent warm room safely and with a minimum transfer of cool air from the cold room to the warm room.
The use of air curtains is one method of allowing a doorway to remain open to traffic while also preventing substantial energy loss between the cold and warm sides of the vestibule. Air curtains generally direct air across the doorway to counter infiltration of warm to the cold room and exfiltration of cold air from the cold room. By way of example, air curtains may direct air horizontally across the doorway or vertically, from an upper portion of the air curtain.
As a safety precaution, it is desirable prevent the formation of fog, ice, and water in the doorway. Ice may form from the mixing of air from the cold and warm sides of the vestibule. The formation of ice at an air curtain depends on the temperature and relative humidity of the cold and warm rooms, and may be characterized by a psychrometric saturation curve. The mixing of air from the relatively warm and cold sides may be characterized by a straight line between points representing the warm side temperature and humidity and the cold side temperature and humidity, which may be plotted on a psychrometric saturation chart along with the curve. Generally, ice may form whenever the mixing line is to the left of, and above, the psychrometric saturation curve, as it is typically plotted.
The formation of ice may be prevented by heating the air discharged from the air curtain. By way of example, the discharged air may be heated to a temperature at a point on the psychrometric saturation chart such that lines to such point from both the cold side and warm side temperature/humidity points remain to the right of, and below, the psychrometric saturation curve, as it is typically plotted.
While avoiding the formation of ice, water, and fog, it is also desirable to operate the air curtain as efficiently as possible, by adding the minimum amount of heat necessary to avoid such problems. With respect to the psychrometric saturation chart, this means keeping the point representing the airstream with the added heat as close to the saturation curve as possible, without causing mixing lines from this point to the cold side and warm side temperature/humidity points to contact or cross the saturation curve.
Because temperature and humidity conditions in the cold and warm side rooms may change, it is desirable in some applications to dynamically condition the discharged air in response to changing conditions. Conventional systems have various shortcomings. Some systems permit operation of the air curtain at points directly on the saturation curve. In changing environments, this permits the formation of ice, water, and fog because the system may not respond as quickly as the conditions change and because the sensors may not be sufficiently accurate for all positions in the vestibule. This is particularly a problem for systems that rely upon mathematical approximations of the psychrometric saturation curve. Also, conventional control systems do not apply to multiple air curtain arrangements. A vestibule may be formed with two or more air curtains positioned adjacent each other across a doorway. Multiple air curtain arrangements—particularly those having three or more air curtains—present a more complex situation than do single air curtains. As a result, conventional systems for dynamically conditioning air do not operate efficiently, or even properly, in multiple air curtain arrangements.